Certain synthetic resins, particularly homopolymers and copolymers of vinyl chloride, which are very widely used because of their low cost and good physical and chemical properties, have a low impact strength at ambient temperatures, at low temperatures, or after aging. The proposal has been made to remedy these faults by incorporating into these resins products, called impact additives, which are polymers with a certain elastomeric character. Of particular interest among these additives are polyacrylates, modified or unmodified, and also statistical or graft copolymers based on butadiene. In particular, U.S. Pat. No. 3,264,373 describes an impact additive for polymer-based resins containing a least 80% by weight of polymerized vinyl chloride: it consists of a copolymer of methyl methacrylate grafted onto a backbone formed of a polymer consisting, by weight, of 1 to 20% butadiene and 99 to 80% of an alkyl acrylate of C.sub.2 to C.sub.12 alkyl, said graft copolymer having a grafted methyl methacrylate content of between 10 and 185% preferably 20-50% by weight of the backbone copolymer. Incorporated in the vinyl chloride polymer resin, in a proportion of 0.5-50 parts by weight for each hundred parts of resin, such an additive produces a substantial increase in the impact strength of the resin, with practically no reduction in its heat distortion temperature.
It has now been found that if, in such an additive, the backbone copolymer is replaced with a similar copolymer that has been cross-linked by means of a small quantity of polyfunctional cross-linking agent, one obtains an impact additive with improved effectiveness.
The impact additive used as described in this invention thus produces synthetic resin compositions which have improved impact strength properties at ambient temperature, at low temperatures, or after aging, as compared to those of comparable synthetic resin compositions incorporating the impact additive described in the above-mentioned U.S. patent.